Lack of association between atopic asthma and polymorphisms of the histamine H1 receptor, histamine H2 receptor, and histamine N-methyltransferase genes

Histamine functions as one of the major mediators in allergic diseases such as rhinitis, asthma, urticaria, and anaphylaxis when released from mast ce lls and basophils after cross-linking surface-bound IgE by allergen, The ef fects of histamine are mediated through three types of receptors, the H1 re ceptor (HRH1), H2 receptor (HRH2) and H3 receptor (Hill 1990). HRH1 mediate s the proinflammatory actions of histamine in the cytokine release and adhe sion process (Bachert 1998; Banu and Watanabe 1999), while HRH2 suppresses the production of the Th1-inducing cytokine interleukin-12, resulting in a shift of the Th1/Th2 balance toward Th2 dominance (van der Pouw Kraan et al . 1998). Furthermore, a linkage study identified Chromosome 3p25, where HRH 1 is localized, as a candidate gene region for atopy in the Japanese popula tion (Yokouchi et al. 1999).